The invention relates to damage detection and structural health evaluation in various structures such as mechanical structures and facilities, buildings, civil engineering works, and aerospace structures.
Damage detection and structural health evaluation on a structure have been performed in various ways. For example, piezoelectric impedance-based damage detection technique is used to detect a damage created in a structure in an early stage. (For example, refer to JP-A 2007-085733, JP-A 2004-028907 and JP-A 2001-099760.) In the piezoelectric impedance-based damage detection technique, a piezoelectric element is affixed on a surface of a structure, and a change in dynamic characteristics of the structure due to a damage created in the structure is detected as a change in electrical impedance of the piezoelectric element in a supersonic range between a few tens kHz to a few hundreds kHz. The inverse of the electrical impedance (or admittance) of the piezoelectric element depends on electrostatic capacitance of the piezoelectric element and driving point mobility of the structure observed from the piezoelectric element. When a damage is created at a structural point near the affixed point of the piezoelectric element, the driving point mobility of the structure is changed largely in a range between a few tens kHz to a few hundreds kHz, so that the electrical impedance (or admittance) of the piezoelectric element is changed largely in the range according to a relationship between the dynamic characteristics of the structure and the electrical impedance (or admittance). Then, it is said that a very small damage is detected sensitively near the affixed point by measuring the electrical impedance of the piezoelectric element. An impedance analyzer or a device dedicated for measuring impedance is used for the measurement of electrical impedance.
However, in the piezoelectric impedance-based damage detection technique, an effect of a damage on high frequency waves is evaluated as a frequency response (impedance). Thus, the dynamical effect due to the damage is observed only as a time average. A damage is viewed as a static one, and for example, various nonlinear effects and interactions with the high frequency waves are neglected at the interface of the damaged area. Further, the evaluation is relative because a damage is detected as a “change” in electrical impedance of the piezoelectric element affixed to the structure before and after the structure is damaged, or a “baseline” data is necessary as a standard for the evaluation. The healthy impedance or admittance as a baseline varies largely with the structure, and it also depends on the affixed point or a size of the piezoelectric element. Therefore, the baseline cannot be predicted with a calculation or the like practically, and it has to be measured actually. This means that structural health cannot be decided with only one measurement in principle, and it is a large problem on the application or operation of the technique.
Further, a damage in an early stage often has a form of a “hidden damage” such as a closed crack or a kissing bond, and if such a damage is not detected, it might be a very great danger on safety management. However, a supersonic wave is transmitted through a damage such as a closed crack, so that it would be difficult to detect such a damage with the piezoelectric impedance-based damage detection technique.
On the other hand, nonlinear wave modulation-based damage detection technique is one of the techniques effective for detecting a “hidden damage” without baseline data in principle. (For example, refer to C. Liang, F. P. Sun, C. A. Rogers, An impedance method for dynamic analysis of active material systems, Journal of Vibration and Acoustics, Transactions of the ASME, Vol. 116, pp. 120-128, 1994; G. Park, H. Sohn, C. R. Farrar, D. J. Inman, Overview of piezoelectric impedance-based health monitoring and path forward, The Shock and Vibration Digest, Vol. 35, No. 6, pp. 451-463, 2003; K. E.-A. Van Den Abeele, P. A. Johnson and A. Sutin, Nonlinear elastic wave spectroscopy (NEWS) techniques to discern material damage, Part I: Nonlinear wave modulation spectroscopy (NWMS), Res Nondestr Eval, Vol. 12, pp. 17-30, 2000; V. Zaitsev, V. Gusev, B. Castagnede and P. Sas, Micro-damage detection using a modulation technique based on dissipative nonlinear effects, Proceedings of Forum Acusticum Sevilla 2002, 2002; and V. Zaitsev and P. Sas, Nonlinear response of a weakly damaged metal sample: a dissipative mechanism of vibro-acoustic interaction, Journal of Vibration and Control, Vol. 6, pp. 803-822, 2000.) In this technique, it is noted that a damage such as a crack created in a structure, a slacked bolt, or detachment at an adhering face accompanies a change in contact state between contacting faces. Then, a change in contact state between the contacting faces due to low frequency dynamic load fluctuations is taken out as amplitude or phase modulation of high frequency waves received from an electromechanical transducer such as a piezoelectric element. If there is no damage, no modulation occurs, and this evaluation is absolute in principle. Thus, structural health can be decided by one data acquisition with nonlinear wave modulation-based damage detection technique. Further, because this technique uses a change in variations of dynamic load at low frequencies exerting a damaged site, it has an advantage that a “hidden damage” can be detected in principle.
However, for nonlinear wave modulation-based damage detection technique, at least two piezoelectric elements are needed for sending and receiving high frequency waves, and this is a problem when the technique is applied to a situation wherein a space for affixing the piezoelectric elements is restricted strictly. Therefore, it is a problem to develop a self-sensing technique using only one piezoelectric element.